Of course, the best way to improve citations is to write good papers which presents sound, interesting, and important results.
However, assuming that one has a nice paper published in an important journal (e.g., PRL, or PRB), what one can do to improve citations, and in general to make people read, or at least aware of, the paper? A partial answer is to upload it on the arXiv, and presents the work to relevant conferences on the topic. Is there something else one can do to advertise a paper, especially online?
PS: I am concerned with hard-sciences, in particular, physics.